Stellar photosphere temperatures

[astrotizio]

Hi all,
Star surface temperature determines the occurrence of photosphere, where radiation can escape from star interior due to diminishing gas ionization and radiation absorption.
If it was true the star photospheres should have the same temperature and Herzsprung-Russell diagram should be only a point.
Actually initial mass should determine the difference, but what is the machanism?
What is wrong in this reasoning.
I refer to cosmic background radiation map that is defined our universe photosphere with an emission temperature of about some thousand degree Kelvin, temperature at which plasma ionization ceases.
Thanks of your time

Tiziano Colombo

 

[D H]

If it was true the star photospheres should have the same temperature

That's obviously not true; we see stars whose equivalent surface temperature ranges from rather cool (red dwarf and red giants) to extremely hot (blue stars).

Actually initial mass should determine the difference, but what is the mechanism?

Stars on the main sequence are in a state of equilibrium, more or less. The energy produced by fusion at the center of the star is typically balanced by the energy emitted from the surface of the star. Internally, stars have a number of negative feedback mechanisms that keep the temperature, pressure, and fusion rates in balance. How this relates to energy emitted from the surface of the star is a bit complex due to the various mechanisms by which energy is transferred outwards from a star's core. Red dwarfs are fully convective throughout. Solar mass stars (~0.5 solar masses to ~1.5 solar masses for high metallicity stars) have a non-convective core surrounded by a convective zone. Larger stars, where the CNO cycle dominates over the *pp* chain, have a convective core surrounded by a non-convective outer layer.

What is wrong in this reasoning?
I refer to cosmic background radiation map that is defined our universe photosphere with an emission temperature of about some thousand degree Kelvin, temperature at which plasma ionization ceases.

What is wrong with your reasoning is that you are extrapolating, and you are doing so across many orders of magnitude. The answer is "don't do that then." In other words, don't extrapolate.

 

[astrotizio]

Thanks for your answer

 

[snorkack]

The transparency of gas depends on its temperature, but not only temperature: it also depends on density (and other factors). Considering just temperature and density, photosphere might happen in low density, high temperature gas, or in high density, low temperature gas. This is what allows stars to have different temperatures.

 

[astrotizio]

This is a convincing answer, thanks

 

[snorkack]

Hi all,
Star surface temperature determines the occurrence of photosphere, where radiation can escape from star interior due to diminishing gas ionization and radiation absorption.
If it was true the star photospheres should have the same temperature and Herzsprung-Russell diagram should be only a point.

Also this would not follow. If all star photospheres had the same temperature, they could still have different luminosities and radii, and form a line rather than a point on Herzsprung-Russell diagram.

 

[astrotizio]

I agree, a point in the H-R diagram was a banal error.
With the same temperature but different radii you obtain different luminosities in the vertical axis, following
L = 4*pi*R^2*T^4

 

[astrotizio]

Sorry in the previous expression i forgot a constant